Manufacture of many articles involves attachment of a relatively pliable layer to a relatively rigid layer. Electrodes constitute one class of such articles.
Electrodes are widely used in many devices that store electrical energy, including primary (non-rechargeable) battery cells, secondary battery cells, fuel cells, and capacitors. Because of various competing performance criteria that need to be considered when designing electrodes, many electrodes are constructed using two or even more constituent materials. One application where such composite electrodes are often used is construction of double layer capacitors, also known as electrochemical capacitors, supercapacitors, and ultracapacitors.
Double layer capacitors employ, as their energy storage elements, electrodes immersed in an electrolytic solution (electrolyte). Typically, a porous separator impregnated with the electrolyte ensures that the electrodes do not come in contact with each other. A double layer of charges is formed at each interface between the solid electrodes and the electrolyte. Double layer capacitors owe their descriptive name to these layers.
In comparison to conventional capacitors, double layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for this volumetric and weight efficiency. First, the width of the charge separation layers is very small, on the order of nanometers. Second, the electrodes can be made from a porous material, having very large effective area per unit volume, i.e., very large normalized effective surface area. Because capacitance is directly proportional to the electrode area, and inversely proportional to the width of the charge separation layer, the combined effect of the narrow charge separation layer and large surface area results in capacitance that is very high in comparison to that of conventional capacitors. High capacitance enables double layer capacitors to receive, store, and release large supplies of electrical energy.
Another important performance parameter of a capacitor is its internal resistance, also known as equivalent series resistance (ESR). Frequency response of a capacitor depends on the characteristic time constant of the capacitor, which is essentially a product of the capacitance and the internal resistance, or RC. To put it differently, internal resistance limits both charge and discharge rates of a capacitor, because the resistance limits the current that flows into or out of the capacitor. Maximizing the charge and discharge rates is important in many applications. In hybrid automotive applications, for example, a capacitor used as the energy storage element powering a vehicle's engine has to be able to provide high instantaneous power during acceleration, and to receive power produced by regenerative braking.
High internal resistance may create heat during both charge and discharge cycles. Heat causes mechanical stresses and speeds up various chemical reactions, thereby accelerating capacitor aging. Moreover, the energy converted into heat is lost, decreasing the efficiency of the capacitor. It is therefore desirable to reduce internal resistance of capacitors.
Active materials used for electrode construction—activated carbon, for example—usually have rather limited specific conductance. Thus, large contact area may be desired to minimize the contact resistance between the electrode and its terminal. The active material may also be too brittle or otherwise unsuitable for directly connecting to terminals. Additionally, the material may have relatively low tensile strength, needing mechanical support in some applications. For these reasons, electrodes typically incorporate current collectors.
A current collector is typically a sheet of conductive material on which the active electrode material is deposited. Aluminum foil is commonly used as the current collector material of an electrode. In one electrode fabrication process, a solvent based electrode film is produced, and then attached to a thin aluminum foil using a wet solvent based adhesive or binder layer. To improve the quality of the interfacial bond between the film of active electrode material and the current collector, the combination of the film and the current collector is processed in a pressure laminator, for example, a calender or another nip. Pressure lamination increases the bonding forces between the film and the current collector, and reduces the equivalent series resistance. After laminating the combination of solvent based electrode film, wet adhesive binder, and current collector are subsequently dried to remove any liquid solvent, lubricants, or impurities.
As has already been mentioned, high capacitances of double layer capacitors result, to a great extent, from the high normalized effective surface area of the active electrode layers. Porosity of the active electrode layer film plays an important role in increasing the effective surface area. Generally, porosity on a small scale level is unchanged when the active electrode film is densified through compaction, for example, through calendering or processing in another kind of high-pressure nip. Because compacting reduces the film's volume while keeping pore surfaces relatively unchanged, the normalized effective surface area is increased. Furthermore, compacting tends to decrease the equivalent series resistance, and possibly also improves structural integrity of the film. For these reasons, current solvent based active electrode films are often compacted before they are attached to current collectors.
The material of a typical active electrode film is compressible and malleable. When the film is processed in a calender, alone, or onto a wet adhesive binder layer, it tends not only to densify through compaction in the direction of pressure application, but also to deform, elongating and widening in the plane transverse to this direction. This is problematic for two reasons. First, densification is reduced, potentially requiring multiple compaction/densification steps. Second, the film may need to be trimmed because of spreading, i.e., because of the elongation and widening. Trimming becomes necessary, for example, when the film spreads beyond the current collector surface, or when the film spreads to the areas of the current collector that need to be connected to other components, such as terminals or other electrodes. The additional compacting and trimming steps increase processing costs and time, and are best reduced or avoided altogether. These problems are not necessarily limited to electrode fabrication, but may be relevant when densifying and laminating other compressible materials.